One of California’s newest reservoirs is set to get 60 percent bigger with the award this week of a Contra Costa Water District construction contract to raise the dam at Los Vaqueros Reservoir by 34 feet.

The expansion is meant to better protect the district’s 550,000 customers from drought, though it also is expected to improve water quality and give the agency more flexibility to ease water diversions at environmentally sensitive times, district officials said.

The Concord-based district, which built the reservoir in the late 1990s, will continue to be the sole owner. But district officials are leaving the door open for other Bay Area agencies to invest in it later, and possibly even further enlarge the reservoir.

“That’s still a possibility,” said Jerry Brown, who was named general manager in September. “This expansion does not prevent that from happening.”

The district still needs a handful of environmental permits before it can begin construction in the spring, but Brown said he does not anticipate problems.

Though environmental groups often oppose new dams, opposition to the Los Vaqueros project has been muted.

“Most of the folks who work on Delta water policy are not anti-dam. We’re anti-dumb dam,” said David Nesmith, facilitator of the Oakland-based California Environmental Water Caucus.

Nesmith said the Los Vaqueros project is attractive because it has very effective screens that prevent fish from being drawn out of the Delta and because it is built in an off-stream valley where it does not have the severe effect on rivers that in-stream dams do, Nesmith said.

Nesmith suggested other East Bay water districts — particularly the East Bay Municipal Utility District and Zone 7 Water Agency — might do well to participate in the enlargement to boost their water supplies.

The Oakland-based EBMUD is considering raising its 1920s-era dam on the Mokelumne River, which Nesmith and other environmentalists strongly oppose.

EBMUD spokesman Charles Hardy said there are unanswered questions about the costs and benefits of his district buying into Los Vaqueros, but he added, “We think it’s possible that something could be worked out.”

Zone 7 spokeswoman Boni Brewer, however, said her district, a water wholesaler that supplies the Dublin, Pleasanton and Livermore area, has sufficient storage in Lake Del Valle and underground.

“We really don’t need additional storage at this time,” Brewer said.

Beginning in about March, Barnard Construction Company will begin cutting down the 220-foot-high dam to expose its core, and then begin raising it back up by the end of the year.

When they are done, the dam will be 34 feet higher and the reservoir capacity will be increased 60 percent, from 100,000 acre-feet to 160,000 acre-feet. By comparison, the district’s customers use about 120,000 acre-feet in a given year.

The expansion will inundate an additional 400 acres, including an area that may be a migratory corridor for endangered San Joaquin kit fox, though it is unknown if or when the foxes last used the area. The district expects to purchase about 5,000 acres of habitat for preservation to offset the loss of habitat for kit fox and other species, Brown said.

During construction, fishing will be closed on the northern part of the lake near the dam but it will mostly remain open on the southern part. However, access to fishing will be limited during a two- or three-month period when the marina on the south side of the lake is moved.

The $35 million construction contract awarded this week is one of several pieces of the overall, $120 million reservoir enlargement.

Planning and environmental studies were paid for by the U.S. Bureau of Reclamation and the state Department of Water Resources, but design and construction are paid for by the water district.

The costs are built into the district’s budget and ratepayers will not see rate increases as a result of the project, Brown said.

The reservoir near Brentwood is one of two regional surface reservoirs built in California during the 1990s. The other is Diamond Valley Lake in Southern California, which is eight times bigger.

In addition, a number of groundwater storage projects have been developed in recent years in the San Joaquin Valley.

The Los Vaqueros expansion was one of five surface storage projects around California chosen for evaluation in 2000. The other four projects are either dead or face very iffy prospects on what remain long roads for them to be built.

In 2004, Contra Costa district voters approved plans to continue exploring the possibility of raising the dam. At the time, the district was looking at expansion up to 500,000 acre-feet, plans that would involve outside partners and require replacing the existing dam.

The Los Vaqueros project is expected to be complete in a year, Brown said.

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